1. Field of the Invention
This invention relates to a tracking servo technique in a driving device for an optical storage medium.
2. Description of Related Art
Generally, in an apparatus for recording or reproducing information on or from a disc such as a Compact Disc (CD), a DVD or a Laser Disc (LD), servo control is needed to accurately read out information from information track (i.e., recording track) formed on the disc. The servo control is to control the speed of an optical pickup for reading information and a disc, the track position and the focus state of a lens by forming a closed feedback loop. For the servo control, an automatic gain control circuit is frequently provided which controls the gain on the basis of the signal inputted by the optical pickup. Particularly, in a technique which performs the automatic gain control by using an external disturbance source, the automatic gain control is effective in measuring the gain of the closed loop to finely control the gain thereby to obtain an appropriate gain value prior to the reproduction operation. Thus, the servo control may be stabilized after the automatic gain control is performed.
However, if there are impediments, such as black dots or surface scratches, on the information track of the disc, defects are introduced to the input signal generated based on the return light supplied from the optical pickup when the light beam passes by such black dot or surface scratch. In such a case, correct input signal cannot be obtained even with the aid of the automatic gain control, and hence the servo control is disturbed. In this view, a conventional apparatus was equipped with a defect detection device for detecting the defect included in the input signal from the optical pickup device. If defect is detected, the drive signal supplied to the driver of the optical pickup device is fixed and held at the DC level before the defect is detected, thereby stabilizing the servo control operation.
In using an optical pickup of three-beam type, the tracking error signal (TE) which is obtained by the subtraction of the return signal of the forward sub-beam from the return signal of the rearward sub-beam is disturbed, and undesired component of horn-like waveform is introduced to the tracking error signal, which is supplied to the driver of the optical pickup device. As a result, the objective lens of the optical pickup is jumped in a certain direction before the above-mentioned holding effect appears. This causes large positional shift of the light beam, which degrades the reproduction performance.
It is an object of the present invention to provide a driving device for an optical storage medium, which is capable of stabilizing the tracking servo operation at the time when defect is detected, even in use of the three-beam type optical pickup.
According to one aspect of the present invention, there is provided a driving device for an optical storage medium, including an optical pickup for reading out information from an information track on the optical storage medium; a tracking error signal generator for generating a tracking error signal from the information read out by the optical pickup; a tracking controller for controlling the optical pickup to follow the information track on the optical storage medium based on the tracking error signal generated by the tracking error signal generator; a disturbance detector for detecting a magnitude of a disturbance of the tracking error signal caused by impediments existing on the optical storage medium; a correction signal generator for generating a correction signal corresponding to the magnitude of the disturbance of the tracking error signal detected by the disturbance detector; and a limiting unit for limiting an operation of the tracking controller in accordance with the correction signal generated by the correction signal generator.
By the driving device for the optical storage medium thus configured, the optical pickup reads out the information from the information track on the optical storage medium, and the tracking error signal is generated from the information thus read out. The optical pickup is controlled to follow the information track on the storage medium based on the tracking error signal. The magnitude of the disturbance of the tracking error signal, caused by the presence of the impediment on the storage medium, is detected to generate the correction signal corresponding to the magnitude of the disturbance. The correction signal is used to limit the movement of the optical pickup following the information track of the storage medium. Therefore, if the tracking error signal is disturbed by the impediment on the storage medium, the unnecessary operation of the optical pickup is limited by the correction signal corresponding to the magnitude of the disturbance, thereby achieving stable tracking servo control.
In a preferred embodiment, the optical pickup irradiates a main-beam on the information track of the optical storage medium and irradiates two sub-beams at two different positions in a tangential direction and a radial direction of the optical storage medium. Therefore, the stable tracking servo control can be achieved even in use of the three-beam type optical pickup.
In a preferred embodiment, the correction signal may have a fixed time width and a voltage level which is variable in accordance with the magnitude of the disturbance of the tracking error signal. In another preferred embodiment, the correction signal may have a fixed voltage level and a time width which is variable in accordance with the magnitude of the disturbance of the tracking error signal. Therefore, even if the disturbance of the tracking error signal is varying due to the variable shapes of the impediments, the stable tracking servo control can be ensured. The disturbance detector may calculate the magnitude of the disturbance of the tracking error signal by integration.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below.